Embodiments of the present embodiments described herein generally relate to implantable medical devices, and more particularly to antennas for use therein.
An implantable medical device (“IMD”) is a medical device that is configured to be implanted within a patient anatomy and commonly employ one or more leads with electrodes that either receive or deliver voltage, current or other electromagnetic pulses (generally “energy”) from or to an organ or tissue for diagnostic or therapeutic purposes. In general, IMDs include a battery, electronic circuitry, such as a pulse generator and/or a processor module, that are hermetically sealed within a metal housing (generally referred to as the “can”), and a microprocessor that is configured to handle radio frequency (RF) communication with an external device, as well as control patient therapy.
IMDs are programmed and monitored by an external programmer or external home-based patient care system. RF circuitry and an antenna are embedded within the housing of the IMD, such as the header or adjacent to the header, to allow data communication with the external device or base system. In general, the IMD communicates bi-directionally with the external programmer or base system using the Medical Implant Communication Service (“MICS”) specification. The MICS specification is defined under 47 C.F.R. 95.601-95.673 Subpart E (incorporated herein by reference) and ETSI EN 301 839-1 (incorporated herein by reference). The MICS protocol uses a frequency band between 402-405 MHz and a transmit power of approximately 25 microwatts.
To conserve batter power, the IMD may enter into a sleep mode after a predetermined period of idle communication. While in the sleep mode, the IMD may disable the RF circuitry that conducts the bi-directional communication, such as a MICS transceiver. The IMD may exit the sleep mode once a wake-up signal from an external device is detected. The wake-up signal is generally an on-off key modulation scheme (OOK) at a high frequency such as 2.4 GHz. The detection of the OOK modulation allows the IMD to detect high power signals without the need for a local oscillator and synthesizer in the receiver.
Problems have arisen in designing the antenna for use in the IMDs. In particular, there can be a loss of RF communication performance due to the reduction in size of the header and the housing (also called the “can” or “case”) of the IMD. Further, attenuation is inherent to the system since the RF signal travels through the lossy human body. Another problem is that two antennas are used and tuned to two operating frequencies (near 400 MHz for bi-directional communication and 2.4 GHz for the wake-up signal), yet the size of the two antennas is limited by the size of the header (at least for devices where the antenna is to be fitted inside the header). Ideally, the antennas should each have a length equal to a quarter of the wave length of the operating frequency (near 400 MHz for bi-directionally communication and 2.4 GHz for the wake-up signal). However, due to the operating frequencies of the MICS protocol it is difficult to design two antennas that both fit within a device header while achieving the length of the operating frequencies needed for the IMD. Hence, for antennas to be housed in the device header, the antennas may be smaller than the quarter wavelength constraint resulting in antenna much smaller than needed for select performance.
Previously, it has been proposed to provide a loop or an inverted E-shaped configuration antenna 102 mounted on the IMD 10. For example, the inverted E-shaped antenna 102 shown in FIG. 1 and described in application titled “INVERTED E ANTENNA WITH CAPACITANCE LOADING FOR USE WITH AN IMPLANTABLE MEDICAL DEVICE”, which is expressly incorporated herein by reference in its entirety. However, such antenna configurations may only be optimized for a single frequency (for example, 400 MHz) degrading the performance or range of operation of the IMD at the alternate operating frequency (for example, 2.4 GHz).
Alternatively, IMDs have been proposed that employ the use of two antennas. FIG. 2 illustrates a conventional inverted E-shaped antenna 122 and a mono-pole antenna 124, having a shorter antenna length optimized for higher frequencies, mounted on an IMD 20. However, the use of two antennas on IMDs is problematic due to space constraints in the header of the IMD and increased manufacturing costs.
Accordingly, there is a need to provide a dual band antenna, particularly for IMD applications, that addresses these and other issues. It is to this end that aspects of the embodiments described herein are generally directed.